Generally, a certain picture on the screen are played by selctive luminescence, when electron beam from electron gun of cathode tube is incident to red, green and blue fluorescent materials arranged in sequence as a stripe or dot type.
Referring to FIG. 1, there is illustrated a general cathode tube.
As shown in FIG. 1, the cathode tube includes neck part(N) which has electron gun in the inside for radiating the electron beam to fluorescent materials, Funnel(N) which is connected with said neck part(N) and is deposited to inner and outer sunface with black lead and Pannel(P) which is connected with said Funnel, deposited with fluorescent material and jointed with shadow mask for passing electron beam selectively.
Black matrix and metal-back are deposited to said fluorescent material. The black Matrix is to increase a luminance and the Metal-back is to increase a contrast.
The cathode tube further includes a deflection Yoke(D) for deflecting an electron beam to each directions of the square panel and anode button (A) for supplying an anode potential of high voltages to the said metal-back.
FIG. 2 is a cross-sectional view of electron gun shown to the FIG. 1.
As shown in FIG. 2, the electron gun includes a cathode structure body 55 for emitting hot electrons when heated by steam, the first electrode 50 for controlling hot electrons emitted from the said cathode unit 55, the second electrode 51 for accelerating the electrons controlled by the first electrode 50, the third electrode 52 for concentrating electron beam of a portion of electrons accelerated by the second electrode 51 to the fluorescent membrane, a shield-cup 53 for shielding the magnetic field by the deflection yoke(D) and a bead glass 9 of insulator for fixing the first, second and third electrodes 50, 51 and 52 to maintain a certain intervals. The first 50, second 51, third 52 and shield cup 53 are alligned at equal axies. A performance of an electron gun is largely affected by the interval between each two electrodes.
The said cathode structure body 55 is established to a front face the first electrode 50 and a vulb stem 56 is attached to a back face of the first electrode 50.
Referring to the FIG. 3, there is illustrated a conventional art of the said cathode structure body 55. The cathode structure body includes a heater 57, a closed sleeve 58 which has said heater 57 contained therein, a base metal 59 attached on said sleeve 58 and hot electron emission material 60 deposited on upper junction surface of said base metal 59.
Of course, each cathode for red, green and blue electron beam exists.
Hereinafter, a cathode structure body for producing electronic beam which is incident to green light-emitting fluorescent material is designated as G-cathode, a cathode structure body is for producing electronic beam which is incident to red and blue light-emitting fluorescent material are designated respectively as R and B-cathode in this specification
The said base metal 59 and hot electron emitting material 60 are heated by heater 57, so that hot electrons are emitted.
It is desired that the cathode tube produces appropriate color displays rapidly when power is supplied to it. This property is related directly to the rising rate of the temperature of R G and B-cathodes as heat by heater is conducted to the base metal. When power is supplied to the cathode tube, initial color on screen is to be displayed with stable white or moment green color, wherein said green color is changed to white color at once, becomes stable. This is called white balance, and the white balance must be maintained during drying time of cathode tube. When the color on screen is to be displayed with white color it means that each cathode currents namely red, green and blue have cathode current of equal values. In addition, when the color on initial screen is displayed with green color it means that the reaching time of electron emission temperature for green cathode is faster than the time for red and blue one. From the above fact, we know that the cathode current level for green color is larger than that for red and blue color. The appearance time of a picture means the time which is taken till an appearance of a picture on screen after power supply to heater. Generally, it is fast and expressed with a time to reach a constant cathode current. Initial cathode current means the cathode current till an appearance of a white color on screen after power supply to heater. Conventional methods in the art teach the reduction of appearance time, by preheating the heater prior to switching it on.
If the above method only is used, the initial color appearing on the screen is not always white or green, the so produced white color is not clear. Additionally, the power consumed during the preheating stage mentioned above is much greater than that consumed in a method without a preheating stage. Nowadays, a method is known whereby to overcome the above mentioned problems wherein the reaching time for electron emitting temperature is reduced for the G-cathode only.
Prior art of U.S. patent application Ser. No. 556184 is a method for by changing a radiation rate. However, this prior art method does not result in the appearance of a steady green color. Also, Japanese Patent publication of application No. 4-18652 describes a method to modify the method described in the above mentioned United States patent. patent application. As shown in FIG. 3, this cathode includes base metal having a different cap lengths. According to the Japanese Patent, the difference in cap length of base metal in each cathodes makes the R, G and B-cathode to have different heat capacities, so that it makes a appearance time of a picture for each cathodes different.
However, said Japanese Patent has the additional limitation of decreasing the heat capacity of the cathode, because the base metal on the closed sleeve acts as a bimetal. Further, it has the disadvantage of the slower appearance of a picture due to the heat generated by the heater for the cathodic heating, is conducted through the cap layer of the closed sleeve, the gas metal layer and the hot electron emitting layer.